An electrostatic protective device is used for protecting electronic equipment from electrostatic damages. More specifically, the term "electrostatic protective device" as used in this specification means a device which is connected in parallel with an IC or an LSI that is to be protected, and remains as an electrical insulator under normal condition (in absence of electrostatic pulses) so as not to affect the circuit, but becomes electrically conductive when electrostatic pulses are applied thereto (S1 in FIG. 4 is on) so as to protect electronic devices such as IC's and LSI's from electrostatic damages.
The devices used for protecting IC's and LSI's from electrostatic damages include varistors, zener diodes, and discharge gap devices, and these different protective devices are selected for different applications. Varistors and zener diodes are known to involve high levels of current leakage. In the case of zener diodes, as they have a polarity, it is necessary to connect two of them in opposite directions in parallel with each other for applications where the electric current can flow in both directions, and the cost is therefore relatively high.
On the other hand, discharge type devices involve virtually no leakage current, and are simple in structure, hence less prone to failures. The discharge voltage can be adjusted by changing the discharge gap, and in the case of a sealed structure, by changing the pressure of the sealed gas and the kind of the sealed gas.
Some of the commercially available devices, for instance the one sold under the tradename of "Dia Surge Protector" by Mitsubishi Material, use a cylindrical ceramic block covered by an electroconductive film. The discharge gap is formed in this film by cutting it with laser, and the entire assembly is sealed inside a glass container.
According to those disclosed in Japanese patent laid-open (kokai) publications Nos. 2-223182, 3-89588, 3-261086, 4-22086 and 5-67851, the discharge gap is formed directly on a printed circuit board.
The discharge gap type devices sealed in containers, which are commercially available, have favorable properties, but are relatively large in size due to their complex structures. It is highly difficult to reduce the size of a discharge gap type device to a level suitable for a small device for surface mount or for a chip device, i.e., to 1 to 2 mm in width, 2 to 4 mm in length, and 1 to 2 mm in size. Furthermore, the need for a large number of different materials for its fabrication is expected to work against the effort to reduce the cost.
Those disclosed in Japanese patent laid-open (kokai) publications Nos. 2-223182, 3-89588, 3-261086, 4-22086 and 5-67851 are based on the concept of forming a discharge gap directly on the printed circuit board. According to the normal method of fabrication, the possible size of the gap is no less than 150 .mu.m, and the dimensional tolerance thereof is typically in the range of .+-.20 to 30 .mu.m. As a matter of fact, the dimension of the discharge gap is given as a few millimeters in Japanese patent laid-open publication No. 2-223182, as 4 mm in Japanese patent laid-open publication No. 2-89588, as 0.5 mm in Japanese patent laid-open publication No. 3-261086, and as 0.15 mm in Japanese patent laid-open publication No. 5-678518. With the size of the discharge gap in such a range, the discharge voltage is so high that the effectiveness of these devices is quite limited, and these devices are not suitable for the protection of IC's and LSI's which are highly vulnerable to electrostatic damages. Presumably, the devices proposed in these Japanese patent publications are intended for electronic equipment involving higher voltage levels than IC's as opposed to the aim of the present invention. Thus, it can be concluded that there have been no prior art which has successfully accomplished the task of the present invention by using the technology of fabricating printed circuit boards.
The relationship betwee ne discharge gap sizes and the actual discharge voltage levels is discussed in the following.
FIG. 3 shows the relationship between the discharge voltage (spark voltage) and the gap between a pair of parallel electrodes. (The graph is based on the equations given in page 221 of "Electrostatic Handbook", edited by Electrostatic Association of Japan, published by Ohm, Jun. 20, 1988.) Even in the case of the discharge gap of 0.15 mm or the smallest discharge gap of all those discussed in the prior Japanese patent publications, FIG. 3 indicates that the discharge voltage between the parallel electrodes will be approximately 1.5 kV. In the case of projection type electrodes, the discharge voltage would be 10 to 20% less than this level, but it still would not be sufficient to protect IC's and LSI's. The lack of precision in the size of the gap also makes the previous proposals unsuitable for the object of the present invention.
The previous proposals also did not address the problem of protecting the discharge gap from environmental effects, but without proper protection from environmental effects, the discharge voltage may change due to the contamination of the surface of the electroconductive elements with the moisture and gases that may be present in the environment. If the discharge gap is coated with resist or the like for its protection, it will fill into the gap, and a substantial change in the discharge voltage will result. Even if the gap were successfully reduced to a sufficiently small level for a satisfactory protective action to be achieved even when the gap is filled with resist (this however is a highly unrealistic assumption because the gap is required to be less than 1 to 2 .mu.m for the gap to demonstrate a desired protective effectiveness), there still remains the problem that the resist filling the gap may be affected by the electric discharges, and the reduction in the electric resistance or even the conductance of the gap can occur.